crypto_rsa.c: CID 1439488: Resource leaks

Coverity report from 18th Sept:

/src/lib/crypt_ops/crypto_rsa.c: 558 in crypto_pk_read_private_key_from_filename()
552     {
553       struct stat st;
554       char *buf = read_file_to_str(keyfile, 0, &st);
555       if (!buf)
556         return -1;
557       if (st.st_size > MAX_PRIVKEY_FILE_LEN)
>>>     CID 1439488:  Resource leaks  (RESOURCE_LEAK)
>>>     Variable "buf" going out of scope leaks the storage it points to.
558         return -1;
559     
560       int rv = crypto_pk_read_private_key_from_string(env, buf,
561                                                       (ssize_t)st.st_size);
562       memwipe(buf, 0, (size_t)st.st_size);
563       tor_free(buf);